1. Field of the Invention
The present invention relates to communication systems and more particularly to telecommunication systems employing central message transmitting stations and Earth orbit relay satellites to send messages to mobile receivers. The invention further relates to a method and apparatus for transferring messages to one or more of a plurality of receivers using narrowband, multiplexed, receiver addresses and channel assignments to reduce signal demodulation requirements for each receiver.
2. Background of the Art
There is an ever increasing need for a variety of half-duplex communication systems to provide message delivery or brief transfers of information from a central control station to a multiplicity of remote system users. There are a substantial number of commercial, governmental, and private applications requiring the delivery of messages to a large number of geographically dispersed terminals, or mobile receivers, often on an irregular basis. Applications for message services include such diverse uses as government services (military, law enforcement, legislative) where secure communications are desired, forestry services, disaster relief or coordination, and commercial transport or message delivery services.
Other examples include the interstate trucking industry where dispatchers wish to communicate short messages to trucks located anywhere in continental U.S. Presently such communication is restricted to periodic telephonic communications between drivers and a central dispatcher or contact person. Currently, truck drivers must call long distance from many remote geographic locations to retrieve messages or to update delivery and pick-up schedules. However, it is hard, if not impossible, for drivers to consistently "call in" at fixed, scheduled, times since telephone services are not always readily available in many areas. The conventional "call in" approach also creates accounting problems and major personal inconvenience by requiring vehicle drivers to use money or charge cards for long distance calls at substantial cost even when no updated message or information is transferred.
Aside from conventional telephone systems, other communication systems have attempted to address the mobile market. Radio telephone, cellular telephone, and portable radio transceivers (CB) are all capable of providing some form of communication between a mobile receiver and a base unit. However, these communication systems suffer from several drawbacks and have proven inadequate as message communication systems for serving a large number of widely dispersed users.
Current mobile communication services operate on a limited number of high frequency, low data rate, channels and have many more potential users than system capacity. Many systems, like cellular telephone, employ frequency reuse across an array of cells to increase capacity, with each cell having a short transmission range to increase frequency reuse and reduce interference. However, these lower power transmissions are more prone to frequency selective fading and signal blocking and require highly mobile users to frequently change channels as new cells are traversed. These systems are also prone to sudden communication loss when no channels are available in a new cell.
Cellular systems economically serve large metropolitan areas but leave many urban and most rural areas without any service or direct coverage. Messages for these areas are switched to conventional telephone or satellite carriers for long distance transfers.
Direct communication, non-cellular, using traditional mobile radio transceivers also requires constant monitoring of a variety of frequencies all of which are crowded with existing traffic. These services, like cellular telephone, are subject to frequent system overload and signal degradation from several interference sources which makes them incapable of handling a large volume of traffic. These transceivers also have a low range imposed by the need to prevent interference with other communication systems.
Communication systems based on Earth orbital relay satellites are proposed increasingly as the new approach to solve many communication problems, especially coverage of geographically diverse, low user density or rural areas. Several systems have been developed to operate through orbital relay satellites and central communication stations. Examples of such systems are found in U.S. Pat. Nos. 4,555,651 and 4,504,946.
However, the high gain, low interference requirements imposed on such systems, especially in regards to adjacent Earth orbit relay satellites or concurrent satellite users, requires the use of large receiver antennas on the order of 4 feet or more in diameter, which eliminates mobile applications. In addition, current satellite systems must contend with low data transfer rates which limit the number of users and amount of data transferred. Otherwise, tracking and demodulating a high data rate satellite signal requires very high speed, complex and expensive, receivers that consume a lot of power for signal scanning and processing even when not receiving messages.
What is needed is a communication system that allows continuous delivery of messages as well as related communication parameters or control information to a large number of users over a large geographical area. The system also needs to be cost effective, simple to operate, maintain, and install, and minimize antenna requirements.